This invention relates to a magnetic resonance imaging apparatus which forms the spectrum or tomographic image of a subject by the use of NMR (nuclear magnetic resonance), ESR (electron spin resonance) or the like. More particularly, it relates to an artifact suppression system for the magnetic resonance imaging apparatus as can suppress artifacts appearing when a movable object outside a field of view has overlapped to a tomographic slice, and so on.
Heretofore, there has been well known a magnetic resonance imaging apparatus wherein radio-frequency magnetic field pulses (hereinbelow, termed "RF pulses") perpendicular to a static magnetic field and gradient magnetic fields of three orthogonal axes (X, Y and Z) are applied to a subject lying within the static magnetic field, and a predetermined number of image data sets strings based on magnetic resonance signals induced from the subject are acquired, whereby the desired tomographic image subject is reconstructed by a Fourier transform.
FIG. 1 is a block diagram showing a conventional magnetic resonance imaging apparatus. Referring to the figure, a subject such as human body 1 is arranged through an examination bed 3 within a static field generator 2 which generates a static magnetic field in a direction of Z axis.
A radio-frequency coil 4 which supplies RF pulses A (radio-frequency energy) on the subject 1 and which receives magnetic resonance signals (for example, NMR signals) B from the subject 1, is connected to a transmitter 7 and a receiver 8 by way of a matching unit 5 and a transmission/reception switching unit 6.
Gradient field coils 9, 11 and 13 for respectively applying gradient magnetic fields G.sub.R, G.sub.P and G.sub.S in the directions of three orthogonal axes to the subject 1 are respectively connected to gradient field power sources 10, 12 and 14. Here, the X-axial gradient field is used as a signal read field G.sub.R for encoding a frequency, the Y-axial gradient field as a phase encode field G.sub.P for affording a phase encode magnitude, and the Z-axial gradient field as a slice field G.sub.S for designating a tomographic slice.
A sequence controller 15 controls in a predetermined sequence, all the devices which include the examination bed 3, transmitter 7, receiver 8, and gradient field power sources 10, 12 and 14. A computer 16 which is connected to the receiver 8 and the sequence controller 15, is furnished with a console 17 for inputting parameters etc. required for the construction of an image This computer 16 generates control data for constructing the image, and it executes arithmetic processing and post-processes of a magnetic resonance signal B. An image display unit 18 for displaying the tomographic image is connected to the console 17.
Now, conventional magnetic resonance imaging which employs the apparatus in FIG. 1 will be explained with reference to a pulse sequence diagram of FIG. 2. Incidentally, the pulse sequence in FIG. 2 is stored as part of a program in the computer 16 in FIG. 1 beforehand and is executed by the sequence controller 15. Illustrated here is a case where the magnetic resonance signals B are received by the spin echo technique, and where the tomographic image is constructed from image data based on the magnetic resonance signals B by the use of the two-dimensional Fourier transform method.
First, the subject 1 is inserted inside the radio-frequency coil 4 and the gradient field coils 9, 11 and 13. The radio-frequency coil 4 and the Z-axial gradient field coil 13 are driven, whereby a radio-frequency field pulse (RF pulse) A1 having a selective frequency and a slice field G.sub.S 1 are applied to the subject 1. The RF pulse A1 turns a magnetization of spins 90.degree. ordinarily, and is called the "90.degree. pulse". Thus, nuclear spins within the desired tomographic slice of the subject 1 are supplied with radio-frequency energy, and the phases of the spins begin to be disordered at the central (peak) position of the RF pulse A1.
Next, the Y-axial gradient field coil 11 is driven to apply the phase encode field G.sub.P and to disorder the phases of spins in the Y-axial direction of the tomographic slice, and the X-axial gradient field coil 9 is switched to apply a signal read field G.sub.R 1.
Subsequently, an RF pulse A2 having a flip angle of 180.degree. is impressed in conjunction with a slice field G.sub.S 2 is applied. On this occasion, for the purpose of putting the phases of the spins in order into a slice direction, the slice field G.sub.S 2 is applied in a form extended as indicated by a hatched part, not in a pulse waveform which is symmetric with respect to the peak of the 180.degree. pulse A2. The pulse area of the hatched part agrees with that of the part of the slice field G.sub.S 1 after the peak of the 90.degree. pulse A1 (a hatched part in the figure).
Lastly, the magnetic resonance signal (a spin echo signal) B is received while a signal read field G.sub.R 2 which is identical in polarity to the field G.sub.R 1 is kept applied. This magnetic resonance signal B is fed from the radio-frequency coil 4 into the computer 16 through the transmission/reception switching unit 6 by way of the receiver 8.
In the signal acquisition sequence, the magnetic resonance signal B becomes its peak value at the point of time at which the pulse areas indicated by the hatched parts of the signal read fields G.sub.R 1 and G.sub.R 2 have agreed, that is, at which an echo time interval TE has lapsed from the peak time of the 90.degree. pulse A1. In addition, the timing at which the magnetic resonance signal B becomes the peak depends upon the application timings of the signal read fields G.sub.R and the RP pulses A, and the 180.degree. pulse A2 is applied so as to ensure its peak at the point of time at which 1/2 of the echo time interval TE has lapsed from the peak time of the 90.degree. pulse A1.
The image data items are acquired in such a way that the magnetic resonance signal B is sampled up to a predetermined number of sampling points during the application of the signal read field G.sub.R 2.
The above signal acquisition sequence is repeated the number of times (the number of signal acquisition) corresponding to a predetermined number of pixels N (for example, 256) while the phase encode magnitude equivalent to the pulse area of the phase encode field G.sub.P is being successively changed at predetermined pitches (refer to broken lines and an arrow). Accordingly, in a case where the number of pixels of the tomographic image which is finally constructed is N.times.N, the number of the sampling points of the magnetic resonance signal B at one time is at least N, and the number of times of the signal acquisition of the magnetic resonance signals B is N.
After executing the N times of signal acquisition sequence, the computer 16 subjects the pulse train of the image data based on the magnetic resonance signals B to the two-dimensional Fourier transform and thus reconstructs the tomographic image of the desired matrix size N.times.N, which is displayed on the image display unit 18.
By way of example, the intensity of the phase encode field G.sub.P for every steps is successively changed so as to satisfy the following equation when .gamma. is let denote the nuclear magnetic resonance ratio, L.sub.P the field of view (hereinbelow, also termed the "FOV" simply) in a phase encode direction, and N.sub.P the matrix size in the phase encode direction: EQU .gamma.L.sub.p .intg./G.sub.p .multidot.dt=2 m.pi. (1)
where m=N.sub.p /2, N.sub.p /2-1, . . . , 1, 0, -1, . . . and -N.sub.p /2+1 Accordingly, the variation .DELTA.G.sub.p of the phase encode field G.sub.p becomes: EQU .intg..DELTA.G.sub.p .multidot.dt=2.pi./(.gamma..multidot.L.sub.p) (2)
The computer 16 subjects the image data items based on the plurality of magnetic resonance signals B to the two-dimensional Fourier transform and reconstructs them as the tomographic image of the desired slice whose matrix size is N.sub.R .times.N.sub.p where N.sub.R is the matrix size of frequency encoding direction.
Besides, in case of multislice imaging, immediately after the pulse sequence in FIG. 2, a signal acquisition sequence is executed using RF pulses A at a different carrier (transmission) frequency, thereby to acquire the magnetic resonance signals B of another slice.